1. Field of the invention
The present invention relates to the field of color displays for computer systems and, more specifically, to the field of graphical color presentation and drawing systems.
2. Prior Art
A number of methods of presentation of color information to display devices are well known in the art. In general, such display devices may be divided into two categories; red-green-blue (RGB) devices and NTSC or similar devices. In a RGB device, color information is presented to a display as three separate units of color information; a first unit of information representing the intensity of the red color gun of the display, a second unit representing the intensity of the green color gun of the display and a third unit representing the intensity of the blue color gun of the display.
NTSC devices (and their equivalents under other standards such as PAL) present color information to a display generally by phase-shifting a waveform some predetermined number of degrees from a reference signal. The color display, such as a television set interprets color based on the number of degrees the waveform is out of phase with the reference. Such systems may further control the intensity of the color by controlling the amplitude of the color signal.
The present invention relates to RGB display devices and colors systems.
In a RGB color system, a display may be controlled by presenting bits of color information to drive digital-to-analog converters which in turn produce three analog color signals which control the red, green and blue guns of a display. Typically, 24 bits of color information may be used; 8 bits representing red, 8 bits representing green and 8 bits representing blue. Using 24 bits of color information allows over 16 million (2.sup.24) colors to be produced.
In a typical computer system employing a color display, a device called a "frame buffer" is utilized. A frame buffer is a memory for storing color information corresponding to each pixel on the display. A frame buffer may store 24 bits of information per pixel and the 24 bits of information may be used to directly control the color display. Such a system is typically termed a direct color system. However, use of a full 24 bit frame buffer required a large amount of memory space and leads to other processing in efficiencies. As an example of the amount of memory space required, many known displays, such as a display which may be utilized with the Macintosh II, have displays a comprising 640.times.480 pixels.
It is known to utilize a frame buffer having less than 24 bits of color information per pixel. Such a system may store for example 1, 2, 4, or 8 bits per pixel for color presentation. The bits of information from the frame buffer are used to address a color look-up table (CLUT) The data outputs of the CLUT are the RGB colors signals or their digital equivalents. The use of the CLUT offers a number of advantages. For example, a smaller amount of memory may be used for a frame buffer and colors on the display may be adjusted by adjusting the data content of the CLUT.
The present invention relates to a method in apparatus for presentation of color information to a display utilizing a color look-up table. Such a device may be termed a CLUT device.
A third method for presentation of color information to a display is commonly termed a fixed device. A fixed device, though similar to a CLUT device in featuring an index frame buffer, does not have a changeable CLUT. An example of a fixed device is the IBM Enhanced Graphics Adapter (EGA) standard.
One objective of the present invention is to develop color graphics capable of producing image-quality graphics, i.e., the ability to display a reasonable likeness of a color photograph in a microcomputer system.
A second objective of the present invention is to avoid speed and performance degradation in the computer system for users not utilizing such image quality graphics. For example, a user utilizing a word processing system has little need for high quality color graphics.
A third objective of the present invention is to allow a user to cut graphics created in a first graphic mode and paste the graphics into a display created in a second graphics mode.
These and other objectives of the present invention are described in more detail with reference to the Detailed Description of the Invention and with reference to the drawings.